I. Field of the Invention
The present invention relates to an active suspension apparatus using hydropneumatic springs for suspending a vehicle body on wheels by utilizing fluid and gas pressures.
II. Description of the Prior Art
In a conventional vehicle suspension damping apparatus wherein shock absorbing/damping force characteristics are variable, a vehicle velocity, a relative displacement of wheels, a relative velocity and an acceleration of a passenger's compartment are used as evaluation functions. The absorbing/damping force characteristics are changed in accordance with the evaluation functions. However, the evaluation functions are determined by specific travel states among various vehicle travel states, and one of motion states and vibrations such as pitch and bounce of the vehicle during starting or braking and rolling during a lane change is controlled on the basis of a single evaluation reference. Therefore, according to the conventional apparatus described above, the control operation is limited to level control or control for a predetermined period of time. As a result, road conditions varying from smooth to rough and vehicle velocities varying from low to high velocity cannot be optimally evaluated, and thus the damping force cannot be optimally controlled.
In the state-of-the-art techniques, a displacement and a velocity of a wheel and the vehicle body relative to each other have been used to evaluate the road surface along which the vehicle travels. In general, the travel state of the vehicle is greatly influenced by road conditions. In particular, when riding comfort and driving stability are to be determined, the travel condition of the vehicle cannot be evaluated since the degree of vibrations at the passenger's position cannot be detected, resulting in inconvenience. Evaluation of the vehicle travel condition is started with absolute acceleration which is closely associated with the driving comfort of driver or passengers. Therefore, an input to the vehicle which is determined by roughness of the road surface as disturbance and actual traveling of the vehicle is evaluated by a relative displacement between the axle and the body and a rate of change in displacement as a function of time. However, decreases in transient vibrations and driving stability at the time when the vehicle travels on a rough road, changes the lane (e.g., the vehicle detours around an obstacle) or rides over a bump cannot be sufficiently detected by the relative displacement and its rate of change described above. The travel state of the vehicle is recognized by the absolute acceleration at the passenger's position in the vehicle compartment, the relative displacement between the axle and the body so as to indicate a degree of input to the vehicle during traveling, and the rate of change in displacement described above must be totally evaluated. The damping forces must be controlled on the basis of such evaluation.
In the conventional apparatus, an acceleration detecting means is arranged at part of the vehicle body to determine the motion and vibrations of the body, thereby detecting the back-and-forth acceleration and deceleration and the vertical acceleration. However, the acceleration detecting means which does not detect the back-and-forth acceleration and which is mounted at a position excluding the passenger's position is subjected to vibrations of the vehicle frame and the constituting components such as auxiliary parts. As a result, vibrations felt by passengers cannot be precisely evaluated, resulting in inconvenience.